Exciter apparatus for impact member solenoid

ABSTRACT

A control for current interruption and performance regulation in an electrical solenoid is disclosed. In one embodiment of the invention, solenoid current magnitude is controlled independently of supply voltage and variation in electrical element characteristics, and current magnitude is determined in a manner enabling simultaneous adjustment of current level, and solenoid force, in a plurality of similar circuits. Means for control of solenoid excitation in response to any of a plurality of solenoid performance properties is also disclosed.

United States Patent lnventors Kenneth E. Jauch 7 3,136,928, 6/1964 Avis317 14s.5 sm 7 3,178,617 4/l965 Coker 317 33 Charl Koeller, Yellow p tOhio 3,l83,830 5/1965 Fisheret al. 3l7/l48.5 x [2 pp N9 863,8243,469,152 9/l969 Bosman 3l7/l48.5 x [2 1 Filed Oct-6, 1969 3,512,047 51970 Garde 317 33 Patent D 14, 1 1 2,997,632 8/l96l Shepard 3l7/l48.5[73] Assrgnee The NationalCash Register Company 3,096,475 7/1963 Brooks317/33 X Dayton, Ohio 3,274,446 9/l966 Nagata'. 317/33 3,437,905 4/1969Healey et al. 323/9 X 3,445,751 5/l969 Easter 317/33 X [54] EXCITERAPPARATUS FOR IMPACT MEMBER 3 473 lo I 9 bl SOLENOID r 6 l0/ 96 Gra3l7/33 X 13 Claims, 3 Drawing Figs. Primary Examiner-Lewis H. MyersAssistant Examiner-Ulysses Weldon [52] U.S.Cl Almmeys Louis Kline andJohn L Callahan [51] lnt.Cl ..l-l0lh 47/32 of Search A onnfor currentinterruption and per- 157; 1 1/ RC. R formance regulation in anelectrical solenoid is disclosed. in one embodiment of the invention,solenoid current magnitude [56] Rderences is controlled independently ofsupply voltage and variation in UNITED STATES PATENTS electrical elementcharacteristics, and current magnitude is 2,978,630 4/1961 De La Tour317/33 determined in a manner enabling simultaneous adjustment of2,997,632 8/l961 Shepard 3l7/l48.5 current level, and solenoid force, ina plurality of similar cir- 3,048,748 8/1962 Carey.... 317/ I57 cuits.Means for control of solenoid excitation in response to 3,096,475 7/1963Brooks 317/33 X any of a plurality of solenoid performance properties isalso 3,125,7l5 3/1964 Brooks 3 l7/33 X disclosed.

Sl/ISO I35\ I49 I33 I36 I2] I47 122 I48 Patented Dec. 14, 1971 3,628,102

[ also INVENTORS I KENNETH E. JAUCH CHARLES W.KOEL ER BY WITNESS gem WMM/w THEIR ATTORNEYS EXCITER APPARATUS FOR IMPACT MEMBER SOLENOIDBACKGROUND OF THE INVENTION l. Field of the Invention This invention isapplicable to the art of solid state electronic controls usable with.the actuating solenoid of various mechanism, including a high-speedprinting mechanism or other electromagnetically driven mechanisms.

The invention further pertains to a solid state feedback control havinga plurality of inputs one of which is capable of limiting conduction inthe solid state members in response to some performance property of thesolenoid and also in response to some external command which isapplicable to a plurality of similar circuits.

2. Description of the Prior Art It is well known in the prior art tocontrol excitation of a solenoid coil by placing a switching circuitcomposed of one or more solid state elements, such as transistors, inthe electrical excitation path of the solenoid coil. In that prior artsolenoid excitation, the magnitude of current flowing in the excitedcoil becomes a function of the power source's voltage amplitude, thevoltage across the switch element, and the impedance of the solenoid.Where precise mechanical action is needed from the solenoid's outputmember, as is true in a modern high speed printer, the dependence ofsolenoid excitation upon variables in the exciting circuit and uponpower source voltage is undesirable; the high-speed printer environmentoften calls for solenoid excitation to be precisely controlled or, insome instances, to be responsive to a performance characteristic of thesolenoid.

The prior art in solenoid excitation also discloses the use of a linearcontrol element to control excitation of a solenoid coil, a commonembodiment of this concept being a vacuum tube having a relay coilconnected into its plate circuit or a transistor amplifier having arelay coil connected to its output. In prior art linear exciters such asthese, the magnitude ofcurrent flowing in the coil is determined by theamplitude of a signal applied to the amplifier's input terminal or bythe power supply's output voltage or by impedance properties of the coilor by a combination of these parameters.

In the present invention, prior art solenoid excitation is improved uponby providing means for the solenoid current to be controlled in a mannerindependent of variations in circuits parameters and also in response tosolenoid performance and under the influence of an'adjustable referencelevel.

SUMMARY OF THE INVENTION In the 'present invention, current flow insolenoid coils is made to depend upon the magnitude of control signalswhich are separate and apart from that conveying the open and closeDESCRIPTION OF THE DRAWING Fig. I shows an essential element blockdiagram of one embodiment of the present invention.

FIG. 2 shows one embodiment of a solenoid excitation circuit madeaccording to the present invention.

FIG. 3 shows a high-speed printer mechanism which is typical ofmechanisms that may have actuation solenoids excited by the circuit ofthe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT .In prior art high-speedprinting mechanisms. it is common practice to control actuation of theelectrical solenoid by means of a transistor switching circuit.According to this prior artmethod of control, the electrical solenoid,the transistor switching member, and the power source are all placed inseries, and suitably timed pulses are applied to the transistor controlelectrode in order that timed excitation of the solenoid isaccomplished.

In the precisely optimized environment of a third or fourth generationhigh-speed printer mechanism, this prior art method for controllingsolenoid excitation is found to have disadvantages making itincompatible with the degree of precision maintainable in other parts ofthe mechanism.

The simple transistor switching circuit as used in prior art printingdevices, while in the turned-on state, is possessed of a voltage drophaving magnitude directly dependent on transistor saturationcharacteristics. In practice, it has been found that productionvariation in transistors and degradation of transistor characteristicsduring operating life make saturation characteristics vary to a degreewhich is unacceptable for a precisely designed mechanism. In one printermechanism, similar to that shown in FIG. 3 ofthe drawing, for example,it is found desirable to maintain current flowing in the solenoid within1 percent of the design value. It is found that transistor saturationcharacteristics when the transistor is placed in a prior art drivingcircuit are alone sufficient to make such l percent variation limitsdifficult to maintain over a long operating life.

Another difficulty often encountered when a conventional switchingcircuit drives a solenoid in a high-speed printer mechanism is thatvariations in the printer power supply voltage are reflected inperformance of the printer. Since the high energy flows demanded of apower supply in a large data processing printer make the supplydifficult to regulate within close voltage tolerance, it is desirablethat some means to be found are making printing performance reasonablyindependent of power supply fluctuation. The conventional solenoiddriving circuit, with its simple series arrangement of power supply,solenoid, and switching member, is incapable of compensating for'powersupply fluctuation, so that both the force developed within the solenoidmember and the time over which this force increases to a maximum may beexpected to vary in response to power supply fluctuations.

Another disadvantage of the conventional prior art solenoid actuatingcontrol is that nomeans is available for adjusting the current in asolenoid on either a group or an individual basis. It is often foundthat tolerances introduced during fabrication of the solenoid and otherportions of the printing mechanism make it desirable to adjust thesolenoid current on an individual basis. It is also often found thattesting of the printer or operationally varying its performance makescollective adjustment of the solenoid currents desirable. In the priorart conventional solenoid driver control, no provision is made foreither individual or collective current adjustment.

Yet another disadvantage of the conventional prior art solenoidactuating control is that no provision is made for having the solenoidexcitation determined by some property of the solenoid's performance, asopposed to being dependent on solenoid current or supply voltage.

In the present invention, there is disclosed a solenoid driver 1 controlwhich overcomes the difficulties encountered with the comprises a linearcircuit. in lieu of saturating the transistors which drive the solenoidcoil, in Fig. 2 these transistors are maintained, after initial turn-on,in the linear,-or active, region, wherein the degree of conduction iscontrollable and wherein components external to the transistors areparamount in controlling the current level.

The essential parts of one embodiment of the present invention are shownin FIG. 1; these parts comprise an amplifiersensing network identifiedby the numeral 145, a source of time command signals 147 capable ofcommanding opening or closure of a solenoid 149 to be driven, a sourceof reference signal 146, and a path 170 for coupling a solenoidperformance signal into the amplifier-sensing network 145.

The amplifier-sensing network 145 in FIG. 1 is composed of threeamplifier transistors 129, 130, and 131 (FIG. 2) connected into aDarlington amplifier; a comparator transistor 125; a solenoid currentsensing resistance 132; a reverse current blocking diode 124; a reactiveelement 127; a voltagelimiting network comprising a resistor 133 and adiode 134; and terminals which are connectable to other portions of themechanism. Among these terminals, the terminal 123 connects to thesource of input or command signals 147; the terminal 126 connects to thesource of reference signal 146; the terminals 136 connect to the drivensolenoid coil 138; and the lead 135 connects to a source of electricalenergy 160. The source 160 is capable of exciting the solenoid and is asource of positive potential direct current energy in the embodimentshown in FIG. 2.

Operation of the circuit shown in FIG. 2 is as follows. The transistors129, 130, and 131 are connected into a conventional Darlington'amplifierhaving common collectors and series-connected emitters and bases.Conduction in this Darlington circuit is initiated, or turned on, by thereceipt of a positive level signal at the terminal 123.

The current flowing into the terminal 123 during turn-on of theDarlington amplifier is small with respect to the current flowing in thesolenoid coil 138, the Darlington amplifier being capable of providing acurrent gain near 50,000. The current flowing into the terminal 123 isderived from the source ofcommand signal 147, which may comprise aconventional switching circuit having a current source resistor 121, aswitching transistor 120, and an output terminal 122 in the presentembodiment. The current flowing between the source ofcommand signal 147and the terminal 123 of the solenoid actuating circuit may be in theorder of one milliamp., a value which is small enough to be easilysupplied by the source of command signal 147, which may be made fromintegrated circuits and can employ either the illustrated junctiontransistors or field effect transistors. 7

Current flowing in the output stage of the Darlington amplifier circuitand the solenoid coil 138 also flows in the series resistor 132. Thisseries resistor 132 has a small value, in the order of one half-ohm orless, so that current levels near ten amperes flowing in the solenoidcoil 138 will produce a volt age drop of five volts or less across theresistor 132. The voltage drop across the resistor 132 is applied to thebase of the transistor 125. Also applied to the transistor 125 at itsemitter terminal is the reference voltage derived from the source 146and appearing on the terminal 126; this reference voltage has a valueequal to the voltage drop expected across the resistor 132 when thedesired current flows in the solenoid coil 138. The voltage applied to'the terminal 126 is therefore determinative of the current level to bereached in the solenoid coil 138 when current limiting operation of theamplifier ensues. Once the voltage appearing across the resistor 132 ismore positive by a few tenths of a volt than the voltage at the terminal126, the transistor 125 begins conducting. Conduction in the transistor125 removes driving current from the input transistor 129 of theDarlington amplifier, by altering the division of current at the node148 This removal of driving current decreases the degree of conductionin the entire amplifier and the current level in the solenoid coil 138.The path through the transistor 125 may thus be considered a feedbackpath which is operative to limit the solenoid current by limitingconduction in the transistors of the Darlington amplifier once thedesired current level has been attained in the solenoid coil 138.

An operating cycle of the circuit shown in F 16. 2 starts with currentbeing supplied intothe terminal 123 to effect turn-on of the Darlingtonamplifier comprising the transistors 129, 130, and 131. Since thecurrent in the resistor 132 will initially be of low amplitude, thetransistor will be nonconducting, and no diversion of the currentflowing into the terminal 123 will occur through the transistor 125.With full driving current from the terminal 123 applied to theDarlington amplifier, saturation or full conduction will occur in itstransistors for a brief time. Depending upon the parameters of thesolenoid circuit, the voltage applied to. the solenoid, and theamplifiercharacteristics, this period of full conduction may last for a timemeasurable in milliseconds, during which essentially the full voltagefrom the power source 160 appears across the solenoid coil 138; thevoltagedrop across the Darlington amplifier is small and on the order ofone to two volts during this full conduction. Once current in thesolenoid coil 138 has reached the magnitude determined by the resistor132 and the voltage at the terminal 126, conduction in the transistor125 commences, and current is diverted away from the Darlingtonamplifier at the node 148 via the path through the diode 124 and thetransistor 125.

Once conduction in the transistor 125 commences, the four transistors ofthe circuit are operating in a closed feedback loop, and, as is typicalof feedback loop circuits, some consideration must be given to thedynamic operating stability of the closed loop. Dynamic stability in thefeedback system comprising the four transistors in FIG. 2 may beanalyzed according to the techniques of Bode and Nyquist, as is commonpractice in the feedback art. in the present circuit, it is found thatsufficient stability results if the frequency response of the Darlingtonamplifier is rolled off at the high frequency end by means of acapacitor 127 placed between the base and the collector of thetransistor 129. This capacitor 127 decreases the gain of the feedbacksystem at high frequencies to a level which is less than unity andthereby afiords dynamic stability. It is also to be observed that thecapacitor 127 acts as a limiter upon the switching speed of theDarlington amplifier when current is first applied at the terminal 123.In practice, a small capacitor isadequate to provide dynamic amplifierstability; however, a larger value may be employed in order that adelayed and slowed solenoid current rise time may be achieved.

Several components are shown in FIG. 2 which are incidental to theoperation of the solenoid driver circuit as explained above. Among thesecomponents are the resistors 128, 139, and connected between base andemitter of the Darlington amplifier transistors. These three resistorsprovide a path for leakage current in each of the amplifier transistors,so that leakage current does not cause conduction during periods whenthe amplifier is to be held in the nonconducting state. With silicontransistors used in the amplifier, leakage currents to be conducted bythese resistors are relatively small, and the value of these resistorsmay be large enough so as not to imburden the amplifying efficiency ofthe Darlington stages during their turned-on operation. Also shown inFIG. 2 is a protective and energy-absorbing network consisting of theresistor and diode network 133-134. which are connected across the coil138 at its terminals; the resistor 133 and the diode 13 4 provide acurrent flow path for the inductive or flyback current flowing in thesolenoid 138 upon removal of the power source by turnoff of theDarlington amplifier. it is well known, in the art of driving aninductive load, that the voltage across terminals of a solenoid memberinstantaneously reverses upon interruption of the applied potential,since energy stored in the inductance of the solenoid attempts tomaintain current flow in the solenoid. Since a positive potential isapplied to the circuit in the FIG. 2 embodiment, the diode 134 in theflyback circuit prevents current flow in the resistor 133 during theinterval when the solenoid 138 is excited by conduction in theDarlington amplifier. Only upon opening of the inductive circuit doesthe diode 134 permit conduction to occur in the resistor 133. Theresistor 133 also offers a means for withdrawing kinetic energy from themoving members of the driven mechanism, since magnetic flux in thesolenoid member couples with these moving members and causes energy fromsuch motion to be converted into electrical form, where it may bedissipated by the resistor 133.

The diode 124 in FIG. 2 is incorporated in the circuit in order thatcurrent from the reference voltage source 146 connected to the terminal126 may not flow into the amplifier via the transistor 125 and currentfrom the resistor 132 may not flow into the amplifier via the transistor125. 1

As was mentioned in introducing the circuit of FIG. 2, it is desirablethat current flow in the solenoid 138 be independent of the solenoidsupply voltage. The circuit of FIG. 2 offers this independence fromsolenoid supply voltage, since the current level maintained by theamplifier-sensing network 145 is determined by comparing the voltagedrop across the resistor 132 to the voltage applied at the terminal 126.In essence, the voltage applied at the terminal 126 acts as a referencein measuring the current flowing in the solenoid 138. Since the voltageapplied at the terminal 126 does function as a reference, it isdesirable that a well regulated and low impedance source be used tosupply the terminal 126. In a practical embodiment wherein there is aplurality of mechanisms and a plurality of solenoid control circuitswithin a single cabinet, the low impedance well regulated source used toexcite the terminal 126 in FIG. 2 may also be employed to excite thecounterpart of the terminal 126 in other solenoid circuits. A feedbackoperational amplifier 142 in FIG. 2 has been found to offer a convenientsource for an adjustable well regulated voltage at the terminal I26.With the proper selection of input and feedback resistances, theoperational amplifier 142 can exhibit output resistances well below avalue of one ohm, so the addition of loads similar to that of theterminal 126 produces negligible effect upon the operational amplifier'soutput signal at the terminal 144.

It is notable that the operational amplifier's output may be madevirtually independent of variations in printer power supply levels bysupplying its input as the terminal 143 from a source (not shown) whichis stabilized by a device such as a Zener diode or some otherstabilizing means.

Since the current level within a solenoid may be changed easily byvarying the voltage applied to the terminal 126 of the amplifier-sensingnetwork 145 in FIG. 2, adjustment of the current level in the solenoidcan be used as a means for adjusting print density when the drivensolenoid actuates a printing mechanism. By causing a low current levelto flow within a printer solenoid, the printing impact may be caused tobe slight and the print density may be caused to be low, while, if thecurrent level is high, a heavy impact and a greater printing densitywill be produced. Low current in the printer solenoid may also be usefulin printing a small number of media copies where it is desired to havelow printing force in order that piercing of the media material notoccur.

Current level in the printing solenoid may be varied in the embodimentshown in FIG. 2 of the drawings by changing the reference voltage levelapplied at the terminal 126. The potentiometer at 141 provides one wayin which this may be accomplished; other ways of varying the voltage atthe terminal 141 are numerous and include changing the value ofresistances in the network surrounding the operational amplifier 142 andvarying the voltage at the terminal 143 of the operational amplifier.

Regardless of how the reference voltage to the terminal 126 is varied,it may be controlled by a potentiometer, and the manipulation of thispotentiometer can provide a convenient operatonaccessible adjustment forprint density and for number of forms to beprinted with the printerhaving the present solenoid control circuit. It is found in practicethat control of the print density by means of controlling solenoidcurrent also affects the timing of the print operation; that is, thetime between application of a signal at the terminal 123 and impact ofthe media material by the printer hammer varies with the adjustment ofcurrent level in the amplifier-sensing network 145. In printers whichare designed'to accommodate this timing variation, control of printdensity and compensation for the number of media thicknesses by means ofcurrent control in the solenoid driver is a convenient and desirablefeature.

The following parts have been found to be suitable for use in thecircuit of FIG. 2: transistor 129, 2N4409; transistor 130, 2N3055;transistor 131, 2N3772; transistor 125, 2N3904; diode 124, 1N46l. Forthe resistances shown in FIG. 2, the following values have been foundsuitable: resistor 128, 33K; resistor 139, 510 ohms; resistor 140, Siohms; resistor 132, 0.4 ohm; resistor 133, 3 ohms; capacitor 127, 680picofarads.

The printing solenoid 138 in FIG. 2 is excited from a power sourcedesignated 160, which has a positive potential of 48 volts in theembodiment shown and is applied via the lead the current level in eachsolenoid driver is adjusted to be near I l amperes at the peak. Thesolenoid has a DC resistance of 1.2 ohms. Solenoid current rise timeoccurs over a period near 0.2 millisecond.

FIG. 3 of the drawings shows one embodiment of a printer mechanism whichmay employ a solenoid control circuit made according to the presentinvention. In FIG. 3, there is shown a printing mechanism having amovable typefont carrier 152, a printing ribbon 150, media to be printedupon 151, a ballistic print hammer 57, and a printing mechanism framemember 160, which is position-adjustable by way of adjusting members161. The numerals 54 in FIG. 3 represent a plurality ofprinter-actuating solenoids as have their exciting coil represented bythe coil 138 in FIG. 2. Also shown in FIG. 3 are a solenoid backstopassembly 24 and a hammer penetration stop assembly 55. Kinetic energyfrom the solenoids 54 is carried to the hammer members 57 by way ofactuating arms such as the one identified by the numeral 17.

The mechanism of FIG. 3 is but one embodiment of a mechanism which mayemploy a solenoid control made in accordance with the present invention.It is intended that the scope of the present invention not be limited toany particular printing mechanism. It is also intended that the scope ofthis invention in many other arts; notable among these are that of highspeed punching of media and that of document and package-handlingequipment. In the high speed punching of media, for instance, it ispossible that a regulating and adjustable solenoid control circuit couldbe used for varying the force with which a punching pin strikes themedia to be punched in order that different media materials beacceptable to the punch and similarly powered. In the document andpackagehandling art, it is possible for a regulating and adjustablesolenoid control circuit to be used for varying the force with which adocument or a package is deflected from one location to another, so thatdifferent trajectories are possible for the deflected article.

Numerous alterationsv are possible for the circuitry shown in FIG. 2 ofthe drawings while yet maintaining the essential features of theinvention. A few of the more obvious of these alternations are citedhere as examples of those which will be suggested to a person skilled inthe art.

In FIG. 2, the use of NPN'junction transistors is shown; it is clearthat PNP-junction transistors could be employed in the circuit withequally desirable results. It is also possible for field effecttransistors to be employed at least in the low current portions of thecircuit.

In FIG. 2, a voltage-limiting network composed of diode 134 andresistance 133 is shown; it is of course possible to replace thisnetwork with many other networks, which range in scope from a singlediode or a single resistance to combinations of diodes, resistances, andcapacitances. The particular network selected will depend upon theperformance characteristics desired in the circuit.

in FIG. 2 and in the text of this disclosure, a Darlington connectedamplifier is employed; for one embodiment of the invention, theDarlington amplifier proved to be a convenient amplifier configuration.However, it is intended that the scope of the invention not be limitedto the Darlington amplifier, since other amplifying connectionsinvolving one or more transistors may be substituted for the Darlingtonamplifier with satisfactory results.

The current-sensing resistance in H6. 2 is shown to be embodied as asingle resistive element 132; in a practical machine, it may bedesirable to embody this resistance as a plurality of series or parallelconnected elements in order that small adjustments to the current flowin an individual solenoid can be made by addition or elimination of anelement. Such an embodiment, for instance, is a convenient way forcompensating small performance difference between adjacent printingmechanisms within a printer.

In FIG. 2, comparison of the solenoid current level to a reference levelis accomplished by means of comparing the voltage developed across theresistor 132 to a reference voltage introduced at the terminal 126 by anexternal source 146, the comparison being made within the transistor125. A person skilled in the art will recognize that other elementconfigurations can be employed to compare a current level to somereference and then adjust the conductivity of a current control element;for instance, the voltage from the resistor 132 could be coupled to someconduction-limiting point in the amplifier circuit through a Zener diodeelement. In this configuration, the Zener diode provides the desiredreference against which signal from the resistor 132 is compared, andthe operational amplifier and associated components are eliminated.

in FIG. 2, a capacitor 127 is connected between the base and collectorterminals of the transistor 129 in order that frequency response of theclosed loop circuit may be controllable. it has been found thatfrequency response of the closed loop circuit may also be controlled byconnecting a capacitor between the base and collector terminals of thecomparator transistor 125 and that the effectiveness of such a capacitorcan be enhanced by placing a resistance in series with the referencesignal from the resistor 132. A capacitor connected across thecomparator transistor [25 is effective to control frequency responsearound the closed loop, just as was the location across the transistor129; however, in the 125 location, this capacitance does not retard theDarlington amplifier's switching speed during initial excitation of thesolenoid a trait which is desirable in some environments; It is clearthat each of these capacitor locations represents just one of aplurality of ways in which frequency response ofa closed loop circuitmay be controlled. Other possible ways for implementing such controlinclude the use of inductance elements within the loop and the use ofshunting capacitance elements between any point having a signal voltageswing and ground.

In FIG. 2, the resistors 128 and 139 are shown connected across thebase-to-emitter junctions of the transistors 129 and 130. It has beenfound that satisfactory or improved operation results in someenvironments if these resistors are replaced with resistors connectedbetween the base of each transistor, 129 and 130, and the emitter of thetransistor 131.

In FIG. 2 and in heretofore describing the invention, means has beenprovided for sensing the current flowing in the sole noid coil [38 andfor coupling a signal representative of that current flow into acomparator circuit and thence into the amplifiers input to regulateconduction in the amplifier.

In a broader sense of the invention, it is not necessary that the signalwhich controls amplifier conduction represent current flow in thesolenoid coil; the signal which controls amplifier conduction mayinstead be derived from one of many per formance properties of thesolenoid which vary during operation. One such property which may besensed is force exerted by the solenoid upon some other member of themechanism. With a signal representative of force coupled into thecomparator, a solenoid which is controlled by the present inventioncould be made to operate in a constant force mode.

Other properties of the solenoid may also be sensed in developing thesignal coupled into the comparator included in these possible otherproperties are:

voltage across the solenoid coil,

magnetomotive force within the solenoid coil,

magnetic flux within the solenoid, and

acceleration of the solenoid output member or some coupled member. Fordeveloping each of these alternate signals, a different form oftransducer will be employed"; for developing a signal representative ofsolenoid acceleration, for example, an accelerometer or force-measuringtransducer would be necessary; for developing a signal representative ofmagnetic flux within the solenoid, a Hall Efiect sensor could beemployed.

The path 170 in FIG. 1 of the drawing is shown in general and in dottedform as an indication that some signal definitive of solenoid-operatingproperties is to be coupled into the amplifier-sensing network from thesolenoid structure. The exact nature of this signal and the manner inwhich it is derived will depend upon the solenoid performance propertywhich is being controlled. It is, of course, possible to combine two ormore of the solenoid-operation-defining signals, such as solenoidcurrent flow and a signal representing solenoid output force, so thatthe solenoids output is controlled in response to more than a singleperformance property.

Through use of the herein-described invention in one embodiment, it hasbeen possible to achieve advancement in the state of the art for ahigh-speed printing mechanism; with this invention, it has been possibleto eliminate the performance degradation often encountered upon heat-upof the coils in a printing solenoid and to eliminate the performancevariations encountered from variation of semiconductor components in theprinter solenoid control circuit and from power supply fluctuations inthe printer mechanism.

What is claimed is: t 1. An excitation control circuit for aprinter-actuating solenoid member ofa high-speed printer mechanism,comprising: a semiconductor amplifier including a plurality ofsemiconductor devices, with the collector electrodes of said transistordevices being connected to a common terminal and thence to agroundableterminal of said printer-actuating solenoid member. said solenoid memberhaving another terminalconnected to a ground referenced source of directcurrent energy and having avoltage limiter circuit connected betweensaid ground referenced source and said groundable terminal,

said semiconductor amplifier having the emitter electrode of a priortransistor device connected to the base electrode of a succeedingtransistor device,

and having an input terminal connected to the base electrode of a firsttransistor device serving as an amplifier input terminal, with saidcommon collector terminal serving as one amplifier output terminal andthe emitter electrode of the final transistor device serving as anotheramplifier output terminal;

a current-sensing resistance of a low value connected between theemitter electrode of the final transistor device and ground so as tohave flowing therethrough the current flowing in said solenoid memberand said final transistor device;

a comparator transistor device having its base electrode connected to ajunction point between said currentsensing resistance and the emitterelectrode of said final transistor device, its emitter electrodeconnected to a source of reference potential, and its collectorelectrode connected through a diode to said amplifier input terminal;

a frequency response determining capacitance element connected betweenthe collector electrode and the base electrode of one transistor devicein said semiconductor amplifiet;

a plurality of leakage current compensating resistances each connectedwith the base electrode of one transistor device so as to conduct awaycollector to base leakage current occurring within said transistordevice; and

a source of timed current signals for commanding desirable opening andclosing of said actuating member, said source of signals being coupledto said amplifier input terminal.

2. Electrical apparatus for effecting intermittent mechanical actuationof a movable print hammer in a printing mechanism; the apparatuscomprising;

a source of direct current energy;

electromagnetic transducer means having a movable member that ismechanically coupled with the movable print hammer, the transducer meansbeing electrically connected with the source of direct current energyand including as parts thereof electrical windings and magnetic fluxconducting structure, the transducer means providing means forconverting energy from the source of direct current energy into energythat moves the print hammer;

cascaded semiconductor linear amplifier means having an output port thatis connected with the source of direct current energy and theelectromagnetic transducer means, for controlling the flow of energyfrom the source of direct current energy to the electromagnetictransducer means, the amplifier means also having an input port forreceiving amplifier controlling input signals, the amplifier beinglinearly responsive to analog signals applied to said input port;

performance sensing means joined to the electromagnetic transducer meansand the amplifier means for generating an analog electrical signalrepresentative of the instantaneous magnetic performance of saidelectromagnetic transducer means;

means for mixing the analog performance sensing means signal with two ormore electromagnetic transducer means control signals, at least one ofwhich is an intermittent control signal, and one of which is an analogtransducer means current controlling signal and for supplying the mixedsignals to the amplifier input port;

whereby the current which excites the print hammer, and

the print hammer, are time responsive to the intermittent control signaland are analog responsive to the analog transducer means currentcontrolling signal.

3. Electrical circuitry in combination with a print hammer electricalsolenoid structure for producing precise intermittent excitationthereof,- said electrical circuitry being composed of:

electrical windings magnetically coupled with said print hammerelectrical solenoid structure, said electrical windings including afirst terminal and a second terminal at the start and finish thereof;

a source of direct current electrical energy capable of exciting saidprint hammer electrical solenoid electrical windings and having a firstoutput terminal and a second output terminal;

a bipolar power transistor having a base terminal, an emitter terminal,and a collector terminal;

a current-sensing resistance element having electrical resistance lessthan one ohm and having a first electrical terminal and a secondelectrical terminal;

means for connecting said source of direct current electrical energy,said electrical windings, said bipolar power transistor, and saidcurrent-sensing resistance element into a closed electrical circuitwherein said electrical windings first terminal is connected to saidsource of directcurrent electrical energy first output terminal, saidelectrical winding second terminal is connected to said bipolar powertransistor collector terminal, said bipolar power transistor emitterterminal is connected with said current-sensing resistance element firstelectrical terminal, and said current-sensing resistance element isconnected with said source of direct current electrical energy secondoutput terminal;

a transistor amplifier including a first junction transistor havingcollector, emitter, and base terminals and a second junction transistorhaving collector, emitter, and base terminals, said transistor amplifierhaving the collectors of the first and second junction transistorsconnected together and connected to the collector terminal of saidbipolar power transistor and having the emitter terminal of the firstjunction transistor connected to the base terminal of the secondjunction transistor and the emitter terminal of the second junctiontransistor connected to the base terminal of said bipolar powertransistor; whereby said transistor amplifier and said bipolar powertransistor are connected into a Darlington circuit;

an adjustable reference signal source capable of generating referencesignals comparable with the voltage developed across saidcurrent-sensing resistance element;

a junction transistor for comparing and mixing signals from saidreference signal source and said current-sensing resistance element,said junction transistor for comparing and mixing signals having a baseterminal that is connected to said current-sensing resistance elementfirst electrical terminal, and an emitter terminal that is connected tosaid adjustable reference signal source, and a collector terminal thatis connected with the base terminal of said transistor amplifier firstjunction transistor; and

timed binary control signal source means connected with said baseterminal of said transistor amplifier first junction transistor forsupplying to said base terminal a binary signal capable of placing saidfirst and second junction transistors in said transistor amplifier andsaid bipolar power transistor in the substantially nonconducting stateand in the controlled conductivity state sequentially, thereby openingand closing said print hammer electrical solenoid structure.

4. Electrical circuitry in combination with a print hammer electricalsolenoid structure as in claim 3 wherein said circuitry also includesenergy dissipating voltage limiting means including an electrical diodeand anelectrical resistance element connected across said electricalwindings first and second terminalsfor dissipating energy stored in themagnetic circuit of said electrical solenoid structure. I

5. Apparatus for effecting precisely controlled intermittent contactbetween a printing font and media to be printed, said apparatuscomprising the combination of:

movable print hammer means for movingsaid printable media into'pressuredcontact with said printing font;

a source of direct current electrical energy;

electrical solenoid means including electrical windings coupled to saidsource of direct current electrical energy and also including mechanicalmeans connected with said movable print hammer means for transducingelectrical energy from said source ofdirect current electrical energyinto mechanical energy vested in said movable print hammer means;

linear amplifier circuit means including an input port and cascadedvariable conductivity elements the final one of which is connectedelectrically via an output port between said source of direct currentelectrical energy and said electrical windings of said electricalsolenoid means for halting and for regulating the instantaneousmagnitude of the flow of electrical energy between said source of directcurrent electrical energy and said electrical windings of saidelectrical solenoid means;

current sensing means connected electrically between said source ofdirect current electrical energy and said electrical windings of saidelectrical solenoid means for generating an electrical signalrepresentative of the instantaneous magnitude of the flow of electricalenergy between said source of direct current electrical energy and saidelectrical windings of said electrical solenoid means;

reference signal source means for producing an electrical signalcomparable with the instantaneous magnitude of said current sensingmeans electrical signal at the instant when said flow of electricalenergy between said source of direct current electrical energy and saidelectrical windings of said electrical solenoid means a attains adesired predetermined magnitude;

digital control signal source means including a source of binaryelectrical signals for determining the printing and the quiescent timeintervals of said movable print hammer means; and

signal comparing and mixing means connected with said current sensingmeans, -said reference signal source means, and saiddigital controlsignal source means and said linear amplifier circuit means input portfor comparing said current sensing means electrical signal with saidreference signal source means electrical signal and generating therefroma difference signal and for mixing said difference signal with saidbinary electrical signals from said digital control signal source meansand for thereby generating and applying to said linear amplifier circuitmeans input port a binary gated negative feedback difference signalcapable of dynamically maintaining said linear amplifier circuit meansvariable conductivity elements in a linear operating region during theprinting time interval of said digital control signal source meansbinary electrical signals. 7

6. Apparatus for effecting precisely controlled intermittent contact asin claim wherein said current sensing means includes an electricalresistance element having resistance less than one ohm.

7. Apparatus for effecting precisely controlled intermittent contact asin claim 5 wherein said final one of said variable conductivity elementsin said linear amplifier circuit means includes a bipolar junction powertransistor which is connected to at least one other bipolarjunctiontransistor in a Darlington circuit configuration.

8. Apparatus for effecting precisely controlled intermittent contact asin claim 5 wherein said signal comparing and mixing means includes abipolar transistor having a base electrode terminal which is connectedwith said current sensing means, an emitter electrode terminal which isconnected with said reference signal source means, and a collectorelectrode terminal which is connected with said linear amplifier circuitmeans input port and to said digital control signal source means. I v

9. Apparatus for effecting rapid precisely controlled simultaneouspressure contact between a medium to be printed and plural locations ona continuously moving printing font. comprising the combination of: i Y

a plurality of movable print hammer means for moving said printablemedium into pressure contact with said continuously moving printingfont;

a source of direct current electrical energy;

plural electrical solenoid means each including a set of elec tricalwindings connected with said source of direct current electrical energyand mechanical means connected with one of said print hammer means fortransducing electrical energy from said source of direct currentelectrical energy into mechanical energy vested in said print hammermeans;

a plurality of linear amplifier circuit means each including an inputport and cascaded variable conductivity elements at least one of whichis connected electrically between said source of direct currentelectrical energy and said electrical windings of one of said electricalsolenoid means for halting and for regulating the instantaneousmagnitude of the flow of electrical energy between said source of directcurrent electrical energy and said electrical windings of saidelectrical solenoid means;

a plurality of current sensing means each connected electrically betweensaid source of direct current electrical energy and said electricalwindings of said electrical solenoid meansefor generating an electricalsignal representative of the instantaneous magnitude of the flow ofelectrical energy between said source of direct current electricalenergy and said electrical windings of said electrical solenoid means;

a single reference signal source means for producing an electricalsignal comparable with the instantaneous magnitude of saidcurrent-sensing mean electrical signal at the instant when said flow ofelectrical energy between said source of direct current electricalenergy and said electrical windings of said electrical solenoid meansattains a desired predetermined magnitude;

a plurality of digital control signal source means each including asource of binary electrical signals for determining the printing and thequiescent time intervals of said movable print hammer means; and

a plurality of signal comparing and mixing means each connected withsaid single reference signal source means,

one of said current sensing means, one of said digital con-- trol signalsource means, and one of said linear amplifier circuit means input portsfor comparing said current sensing means electrical signal with saidreference signal source means electrical signal and generating therefroma difference signal and for mixing said difference signal with saidbinary electrical signals from said digital control signal source meansand for thereby generating and applying to said linear amplifier circuitmeans input port a binary gated negative feedback difference signalcapable of dynamically maintaining said linear amplifier circuit meansvariable conductivity elements in a linear operating region during theprinting time interval of said digital control signal source meansbinary electrical signals. l0. Exciter apparatus for inducing a rapidenergy controlled pressure contact between imprintablepaper and acontinuously moving printing typefont, the exciter apparatus comprisingthe combination of: y

print hammer means including a movable ballistic print hammer forbringing said imprintable paper and said continuously moving printingtypefont together in a pressured printing relationship; v 4 I electricalenergy to mechanical energy transducer apparatus including an electricalsolenoid having electrical windings and a mechanical connection withsaid print hammer means for converting bursts of electrical energy intobursts of kinetic energyyested in said movable bal-' listic print hammerof said print hammer means; I an electronic amplifier including at leasttwo bipolar junction transistors having commonly connected collectorterminals which are in turn connected to a first amplifier outputterminal and having series-connected emitter and base terminals whereinan emitter terminal of a prior transistor is connected with a baseterminal of a next succeeding transistor and having a base terminal ofthe first transistor connected to an amplifier input terminal and havingan emitter terminal of the last transistor connected to a secondamplifier output terminal; current-sensing means including an electricalresistance having a value less than one ohm and having one terminalthereof connected to said second amplifier output terminal forgenerating a voltage signal proportional in magnitude to the currentflowing in said current sensing means and said electronic amplifier;direct current energy source means for exciting said electri calsolenoid; connecting means for connecting said electrical solenoid ofsaid electrical energy to mechanical energy transducer apparatus. saidfirst and second amplifier output terminals of said electronicamplifier, said current sensing means, and said direct current energysource meansinto a series circuit wherein excitation current for saidelectrical solenoid of said electrical energy to mechanical energytransducer apparatus flows;

frequency response determining means connected with the referencepotential source means for generating a voltage signal comparable withsaid voltage signal generated by said electrical resistance of saidcurrent-sensing means when a predetermined current flows therein;

negative feedback means including a comparator transistor of thejunction type having a base terminal that is connected to said secondamplifier output terminal of said electronic amplifier and an emitterterminal connected to said reference potential source means and acollector terminal connected with said amplifier input terminal forcomparing said current sensing means voltage signal with said referencepotential source means voltage signal and for generating from saidsignals as a negative feedback signal capable of dynamically maintainingsaid electronic amplifier in a conducting but nonsaturated state andmaintaining said current sensing means voltage signal comparable withsaid reference potential source means voltage signal;

. signal transmission path of said electronic amplifier for limiting thehigh-frequency response of said electronic amplifier to a frequency bandwherein said amplifier is dynamically stable;

leakage current compensating means connected with at least one of saidbipolar junction transistors in said electronic amplifier for precludingleakage current conduction in said electronic amplifier; and

digital signal source means including a source of digital signalsconnected with said amplifier input and capable of overcoming saidnegative feedback signal generated by said comparator transistor in saidnegative feedback means for terminating conductivity in said electronicamplifier between actuations of said print hammer means.

11. Apparatus for controlling the flow of electrical energy between asource of direct current energy and a print hammer magnetic actuator,said apparatus comprising:

ing the emitter-to-collector current flowpath therein for completing aseries electrical circuit that also includes said source of directcurrent energy, said print hammer magnetic actuator and said currentsensing resistance;

an adjustable source of reference signals capable of generating voltagesignals comparable with voltage signals appearing across saidcurrent-sensing resistance when a predetermined magnitude of printhammer magnetic actuator current flows therein;

a linear mode current amplifier circuit having input and output portsand including at least one linear mode amplifier transistor, saidcurrent amplifier output port being connected to said bipolar junctionpower transistor base terminal;

a junction transistor having a base terminal connected to said currentsensing resistance element, an emitter terminal connected to-saidadjustable source of reference signals, and a collector terminalconnected via a signal path with said input port of said linear modecurrent amplifier circuit; and

a switching transistor source of digital control signals connected withsaid linear mode current amplifier circuit input port for controllingthe conducting and nonconducting time intervals of said bipolar junctionpower transistor,

whereby said junction transistor, said bipolar junction powertransistor, and said linear mode current amplifier circuit are containedwithin a linear closed loop negative feedback circuit.

12. Apparatus for controlling the flow of electrical energy as in claim11 wherein said signal path connecting said junction transistorcollector terminal with said input port of said linear mode amplifiercircuit includes a semiconductor diode element.

13. Apparatus for controlling the flow of electrical energy as in claim11 wherein said linear mode current amplifier circuit includes frequencyresponse controlling means for limiting the high-frequency response ofsaid amplifier circuit and transistor leakage current-controlling meansfor nullifying the effect of transistor leakage currents in saidamplifier.

1. An excitation control circuit for a printer-actuating solenoid memberof a high-speed printer mechanism, comprising: a semiconductor amplifierincluding a plurality of semiconductor devices, with the collectorelectrodes of said transistor devices being connected to a commonterminal and thence to a groundable terminal of said printer-actuatingsolenoid member, said solenoid member having another terminal connectedto a ground referenced source of direct current energy and having avoltage limiter circuit connected between said ground referenced sourceand said groundable terminal, said semiconductor amplifier having theemitter electrode of a prior transistor device connected to the baseelectrode of a succeeding transistor device, and having an inputterminal connected to the base electrode of a first transistor deviceserving as an amplifier input terminal, with said common collectorterminal serving as one amplifier output terminal and the emitterelectrode of the final transistor device serving as another amplifieroutput terminal; a current-sensing resistance of a low value connectedbetween the emitter electrode of the final transistor device and groundso as to have flowing therethrough the current flowing in said solenoidmember and said final transistor device; a comparator transistor devicehaving its base electrode connected to a junction point between saidcurrent-sensing resistance and the emitter electrode of said finaltransistor device, its emitter electrode connected to a source ofreference potential, and its collector electrode connected through adiode to said amplifier input terminal; a frequency response determiningcapacitance element connected between the collector electrode and thebase electrode of one transistor device in said semiconductor amplifier;a plurality of leakage current compensating resistances each connectedwith the base electrode of one transistor device so as to conduct awaycollector to base leakage current occurring within said transistordevice; and a source of timed current signals for commanding desirableopening and closing of said actuating member, said source of signalsbeing coupled to said amplifier input terminal.
 2. Electrical apparatusfor effecting intermittent mechanical actuation of a movable printhammer in a printing mechanism; the apparatus comprising; a source ofdirect current energy; electromagnetic transducer means having a movablemember that is mechanically coupled with the movable print hammer, thetransducer means being electrically connected with the source of directcurrent energy and including as parts thereof electrIcal windings andmagnetic flux conducting structure, the transducer means providing meansfor converting energy from the source of direct current energy intoenergy that moves the print hammer; cascaded semiconductor linearamplifier means having an output port that is connected with the sourceof direct current energy and the electromagnetic transducer means, forcontrolling the flow of energy from the source of direct current energyto the electromagnetic transducer means, the amplifier means also havingan input port for receiving amplifier controlling input signals, theamplifier being linearly responsive to analog signals applied to saidinput port; performance sensing means joined to the electromagnetictransducer means and the amplifier means for generating an analogelectrical signal representative of the instantaneous magneticperformance of said electromagnetic transducer means; means for mixingthe analog performance sensing means signal with two or moreelectromagnetic transducer means control signals, at least one of whichis an intermittent control signal, and one of which is an analogtransducer means current controlling signal and for supplying the mixedsignals to the amplifier input port; whereby the current which excitesthe print hammer, and the print hammer, are time responsive to theintermittent control signal and are analog responsive to the analogtransducer means current controlling signal.
 3. Electrical circuitry incombination with a print hammer electrical solenoid structure forproducing precise intermittent excitation thereof, said electricalcircuitry being composed of: electrical windings magnetically coupledwith said print hammer electrical solenoid structure, said electricalwindings including a first terminal and a second terminal at the startand finish thereof; a source of direct current electrical energy capableof exciting said print hammer electrical solenoid electrical windingsand having a first output terminal and a second output terminal; abipolar power transistor having a base terminal, an emitter terminal,and a collector terminal; a current-sensing resistance element havingelectrical resistance less than one ohm and having a first electricalterminal and a second electrical terminal; means for connecting saidsource of direct current electrical energy, said electrical windings,said bipolar power transistor, and said current-sensing resistanceelement into a closed electrical circuit wherein said electricalwindings first terminal is connected to said source of direct currentelectrical energy first output terminal, said electrical winding secondterminal is connected to said bipolar power transistor collectorterminal, said bipolar power transistor emitter terminal is connectedwith said current-sensing resistance element first electrical terminal,and said current-sensing resistance element is connected with saidsource of direct current electrical energy second output terminal; atransistor amplifier including a first junction transistor havingcollector, emitter, and base terminals and a second junction transistorhaving collector, emitter, and base terminals, said transistor amplifierhaving the collectors of the first and second junction transistorsconnected together and connected to the collector terminal of saidbipolar power transistor and having the emitter terminal of the firstjunction transistor connected to the base terminal of the secondjunction transistor and the emitter terminal of the second junctiontransistor connected to the base terminal of said bipolar powertransistor; whereby said transistor amplifier and said bipolar powertransistor are connected into a Darlington circuit; an adjustablereference signal source capable of generating reference signalscomparable with the voltage developed across said current-sensingresistance element; a junction transistor for comparing and mixingsignals from said reference signal source and said current-sensingresistance element, said junction transistor for comparing and mixingsignals having a base terminal that is connected to said current-sensingresistance element first electrical terminal, and an emitter terminalthat is connected to said adjustable reference signal source, and acollector terminal that is connected with the base terminal of saidtransistor amplifier first junction transistor; and timed binary controlsignal source means connected with said base terminal of said transistoramplifier first junction transistor for supplying to said base terminala binary signal capable of placing said first and second junctiontransistors in said transistor amplifier and said bipolar powertransistor in the substantially nonconducting state and in thecontrolled conductivity state sequentially, thereby opening and closingsaid print hammer electrical solenoid structure.
 4. Electrical circuitryin combination with a print hammer electrical solenoid structure as inclaim 3 wherein said circuitry also includes energy dissipating voltagelimiting means including an electrical diode and an electricalresistance element connected across said electrical windings first andsecond terminals for dissipating energy stored in the magnetic circuitof said electrical solenoid structure.
 5. Apparatus for effectingprecisely controlled intermittent contact between a printing font andmedia to be printed, said apparatus comprising the combination of:movable print hammer means for moving said printable media intopressured contact with said printing font; a source of direct currentelectrical energy; electrical solenoid means including electricalwindings coupled to said source of direct current electrical energy andalso including mechanical means connected with said movable print hammermeans for transducing electrical energy from said source of directcurrent electrical energy into mechanical energy vested in said movableprint hammer means; linear amplifier circuit means including an inputport and cascaded variable conductivity elements the final one of whichis connected electrically via an output port between said source ofdirect current electrical energy and said electrical windings of saidelectrical solenoid means for halting and for regulating theinstantaneous magnitude of the flow of electrical energy between saidsource of direct current electrical energy and said electrical windingsof said electrical solenoid means; current sensing means connectedelectrically between said source of direct current electrical energy andsaid electrical windings of said electrical solenoid means forgenerating an electrical signal representative of the instantaneousmagnitude of the flow of electrical energy between said source of directcurrent electrical energy and said electrical windings of saidelectrical solenoid means; reference signal source means for producingan electrical signal comparable with the instantaneous magnitude of saidcurrent sensing means electrical signal at the instant when said flow ofelectrical energy between said source of direct current electricalenergy and said electrical windings of said electrical solenoid meansattains a desired predetermined magnitude; digital control signal sourcemeans including a source of binary electrical signals for determiningthe printing and the quiescent time intervals of said movable printhammer means; and signal comparing and mixing means connected with saidcurrent sensing means, said reference signal source means, and saiddigital control signal source means and said linear amplifier circuitmeans input port for comparing said current sensing means electricalsignal with said reference signal source means electrical signal andgenerating therefrom a difference signal and for mixing said differencesignal with said binary electrical signals from said digital controlsignal source means and for thereby generating and applying to saidlinear amplifier circuit means Input port a binary gated negativefeedback difference signal capable of dynamically maintaining saidlinear amplifier circuit means variable conductivity elements in alinear operating region during the printing time interval of saiddigital control signal source means binary electrical signals. 6.Apparatus for effecting precisely controlled intermittent contact as inclaim 5 wherein said current sensing means includes an electricalresistance element having resistance less than one ohm.
 7. Apparatus foreffecting precisely controlled intermittent contact as in claim 5wherein said final one of said variable conductivity elements in saidlinear amplifier circuit means includes a bipolar junction powertransistor which is connected to at least one other bipolar junctiontransistor in a Darlington circuit configuration.
 8. Apparatus foreffecting precisely controlled intermittent contact as in claim 5wherein said signal comparing and mixing means includes a bipolartransistor having a base electrode terminal which is connected with saidcurrent sensing means, an emitter electrode terminal which is connectedwith said reference signal source means, and a collector electrodeterminal which is connected with said linear amplifier circuit meansinput port and to said digital control signal source means.
 9. Apparatusfor effecting rapid precisely controlled simultaneous pressure contactbetween a medium to be printed and plural locations on a continuouslymoving printing font, comprising the combination of: a plurality ofmovable print hammer means for moving said printable medium intopressure contact with said continuously moving printing font; a sourceof direct current electrical energy; plural electrical solenoid meanseach including a set of electrical windings connected with said sourceof direct current electrical energy and mechanical means connected withone of said print hammer means for transducing electrical energy fromsaid source of direct current electrical energy into mechanical energyvested in said print hammer means; a plurality of linear amplifiercircuit means each including an input port and cascaded variableconductivity elements at least one of which is connected electricallybetween said source of direct current electrical energy and saidelectrical windings of one of said electrical solenoid means for haltingand for regulating the instantaneous magnitude of the flow of electricalenergy between said source of direct current electrical energy and saidelectrical windings of said electrical solenoid means; a plurality ofcurrent sensing means each connected electrically between said source ofdirect current electrical energy and said electrical windings of saidelectrical solenoid means for generating an electrical signalrepresentative of the instantaneous magnitude of the flow of electricalenergy between said source of direct current electrical energy and saidelectrical windings of said electrical solenoid means; a singlereference signal source means for producing an electrical signalcomparable with the instantaneous magnitude of said current-sensingmeans electrical signal at the instant when said flow of electricalenergy between said source of direct current electrical energy and saidelectrical windings of said electrical solenoid means attains a desiredpredetermined magnitude; a plurality of digital control signal sourcemeans each including a source of binary electrical signals fordetermining the printing and the quiescent time intervals of saidmovable print hammer means; and a plurality of signal comparing andmixing means each connected with said single reference signal sourcemeans, one of said current sensing means, one of said digital controlsignal source means, and one of said linear amplifier circuit meansinput ports for comparing said current sensing means electrical signalwith said reference signal source means electrical signal and generatingtherefrom a difference signal And for mixing said difference signal withsaid binary electrical signals from said digital control signal sourcemeans and for thereby generating and applying to said linear amplifiercircuit means input port a binary gated negative feedback differencesignal capable of dynamically maintaining said linear amplifier circuitmeans variable conductivity elements in a linear operating region duringthe printing time interval of said digital control signal source meansbinary electrical signals.
 10. Exciter apparatus for inducing a rapidenergy controlled pressure contact between imprintable paper and acontinuously moving printing typefont, the exciter apparatus comprisingthe combination of: print hammer means including a movable ballisticprint hammer for bringing said imprintable paper and said continuouslymoving printing typefont together in a pressured printing relationship;electrical energy to mechanical energy transducer apparatus including anelectrical solenoid having electrical windings and a mechanicalconnection with said print hammer means for converting bursts ofelectrical energy into bursts of kinetic energy vested in said movableballistic print hammer of said print hammer means; an electronicamplifier including at least two bipolar junction transistors havingcommonly connected collector terminals which are in turn connected to afirst amplifier output terminal and having series-connected emitter andbase terminals wherein an emitter terminal of a prior transistor isconnected with a base terminal of a next succeeding transistor andhaving a base terminal of the first transistor connected to an amplifierinput terminal and having an emitter terminal of the last transistorconnected to a second amplifier output terminal; current-sensing meansincluding an electrical resistance having a value less than one ohm andhaving one terminal thereof connected to said second amplifier outputterminal for generating a voltage signal proportional in magnitude tothe current flowing in said current sensing means and said electronicamplifier; direct current energy source means for exciting saidelectrical solenoid; connecting means for connecting said electricalsolenoid of said electrical energy to mechanical energy transducerapparatus, said first and second amplifier output terminals of saidelectronic amplifier, said current sensing means, and said directcurrent energy source means into a series circuit wherein excitationcurrent for said electrical solenoid of said electrical energy tomechanical energy transducer apparatus flows; reference potential sourcemeans for generating a voltage signal comparable with said voltagesignal generated by said electrical resistance of said current-sensingmeans when a predetermined current flows therein; negative feedbackmeans including a comparator transistor of the junction type having abase terminal that is connected to said second amplifier output terminalof said electronic amplifier and an emitter terminal connected to saidreference potential source means and a collector terminal connected withsaid amplifier input terminal for comparing said current sensing meansvoltage signal with said reference potential source means voltage signaland for generating from said signals as a negative feedback signalcapable of dynamically maintaining said electronic amplifier in aconducting but non-saturated state and maintaining said current sensingmeans voltage signal comparable with said reference potential sourcemeans voltage signal; frequency response determining means connectedwith the signal transmission path of said electronic amplifier forlimiting the high-frequency response of said electronic amplifier to afrequency band wherein said amplifier is dynamically stable; leakagecurrent compensating means connected with at least one of said bipolarjunction transistors in said electronic amplifier for precluding leakagecurrent conduction in said electronic amplifier; anD digital signalsource means including a source of digital signals connected with saidamplifier input and capable of overcoming said negative feedback signalgenerated by said comparator transistor in said negative feedback meansfor terminating conductivity in said electronic amplifier betweenactuations of said print hammer means.
 11. Apparatus for controlling theflow of electrical energy between a source of direct current energy anda print hammer magnetic actuator, said apparatus comprising: acurrent-sensing resistance element having electrical resistance lessthan one ohm; circuit means including a bipolar junction powertransistor having emitter, base, and collector terminals and includingthe emitter-to-collector current flow path therein for completing aseries electrical circuit that also includes said source of directcurrent energy, said print hammer magnetic actuator and said currentsensing resistance; an adjustable source of reference signals capable ofgenerating voltage signals comparable with voltage signals appearingacross said current-sensing resistance when a predetermined magnitude ofprint hammer magnetic actuator current flows therein; a linear modecurrent amplifier circuit having input and output ports and including atleast one linear mode amplifier transistor, said current amplifieroutput port being connected to said bipolar junction power transistorbase terminal; a junction transistor having a base terminal connected tosaid current sensing resistance element, an emitter terminal connectedto said adjustable source of reference signals, and a collector terminalconnected via a signal path with said input port of said linear modecurrent amplifier circuit; and a switching transistor source of digitalcontrol signals connected with said linear mode current amplifiercircuit input port for controlling the conducting and nonconducting timeintervals of said bipolar junction power transistor, whereby saidjunction transistor, said bipolar junction power transistor, and saidlinear mode current amplifier circuit are contained within a linearclosed loop negative feedback circuit.
 12. Apparatus for controlling theflow of electrical energy as in claim 11 wherein said signal pathconnecting said junction transistor collector terminal with said inputport of said linear mode amplifier circuit includes a semiconductordiode element.
 13. Apparatus for controlling the flow of electricalenergy as in claim 11 wherein said linear mode current amplifier circuitincludes frequency response controlling means for limiting thehigh-frequency response of said amplifier circuit and transistor leakagecurrent-controlling means for nullifying the effect of transistorleakage currents in said amplifier.